Distributed Raman fiber amplification (directly using the transmission fiber as the gain medium) is a powerful technique to improve the optical signal to noise ratio margin of long haul wavelength-division-multiplexing (WDM) systems used, for example, for long-haul telecommunications transmission. Discrete Raman fiber amplifiers (RFA) using dispersion-compensation fiber (DCF), special highly nonlinear fiber or other optical fiber with similar characteristics as the gain mediums have also received much attention in recent years due to their advantage of flexible bandwidth design with relatively low noise characteristics.
Referring to FIG. 1(a), a distributed/discrete RFA may be configured as a forward-pumped RFA (the signal light 100 and the Raman pump light (m pumps 103) co-propagate in the fiber 101 of WDM system 102 or, referring to FIG. 1(b), may be configured as a backward-pumped RFA (the signal light 100 and the Raman pump light 103 counter-propagate in the fiber 101)). Similar reference characters are used throughout the figures to represent similar elements. The first number of a reference numeral refers to the drawing number where that element first appears. In a common Raman/Erbium Doped Fiber Amplifier (EDFA) hybrid WDM system, usually only backward-pumped distributed RFAs are used, but both forward-pumped RFAs (distributed) and backward-pumped RFAs (distributed and discrete) may be used in a Raman-only WDM system. In Europe, rare earth doped optical fiber, other than Erbium doped, such as Praseodymium doped optical fibers have been utilized.
Referring to FIG. 1(c), the flat gain bandwidth provided by a single Raman pump is only about 10 nm. The dotted line arrows of FIG. 1(c) point from a pump to its representative energy transfer characteristic shown in dashed line. But the Raman gain spectrum can be broadened by providing pump energies at a plurality of different wavelengths as is illustrated in FIG. 1(c) where three Raman pumps are shown. The broadening is shown by the composition of the energy transfer characteristics of the three depicted pumps as a composite Raman gain profile in solid line having a much wider, flat bandwidth gain characteristic. While only three pumps are shown, for a typical C/L-band WDM system, usually four Raman pumps at different wavelengths are required to achieve a flat gain spectrum over a broad bandwidth. More pumps may be used to similarly achieve still wider bandwidth than C/L band.
However, a RFA with a constant pump level will not produce a well-controlled output signal in response to large variations in the input signal level, which could be caused, for example, by channel add/drop, accidental fiber cuts or upstream amplifier failure among other similar events in a photonic network using reconfigurable OADMs (optical add-drop multiplexer). With fixed pump powers, if the input signal power decreases suddenly due to channel drops, the Raman gain becomes too high, and the output power per channel increases more than desired. On the other hand, if the input signal power suddenly increases due to the addition of new channels, the Raman pumps become depleted, which causes the output power per channel to decrease more than desired. In our co-pending, concurrently filed U.S. patent application Ser. No. 11/381,244, filed, May 2, 2006, which application is incorporated by reference as to its entire contents, there is described a method and apparatus for quickly controlling tilt transients.
To stabilize the gain profiles of a RFA during channel add/drop, several control methods are known. These include an all-optical gain-clamping technology described by inventor Xiang Zhou and others and a PID (proportional-integral-derivative) based feedback pump power control technique described by C. J. Chen et al., “Control of Transient Effects in Distributed and Lumped Raman Amplifier,” Electronic Letters, pp. 1304-05, October, 2001; L. L. Wang et al., “Gain Transients in Co-pumped and Counter-pumped Raman Amplifiers,” IEEE Photonics Technology Letter, pp. 664-666, May, 2003, and M. Karasek et al., “Modeling of a Pump-power-controlled Gain-locking System for multi-pump Wideband Raman Fiber Amplifiers,” IEEE Proceedings—Optoelectronics, pp. 74-80, April, 2004, P. M. Reepschlager et al. (EP 1248334), and C. J. Chen et al. (U.S. Pat. No. 6,441,950). Another method is a pump power control technique using a half-analytical Raman power model described by P. Kim and N. Park, “Semi-analytic Dynamic Gain-clamping Method for the Fiber Raman Amplifier,” IEEE Photonics Technology Letter, pp. 768-770, April, 2005. For the three known methods, the first method is only applicable for a discrete RFA and will also degrade the noise performance, and the third method is too slow (in the millisecond to second range) to be used to suppress Raman transient effects (tens to hundreds of μs for a backward-pumped RFA and tens to hundreds of ns for a forward-pumped RFA).
For the PID-based feedback control method (i.e. method 2), usually the Raman pumps are divided into several wavelength groups (at least two groups for a typical C-/L band WDM system) and the power adjustments of different pump groups are controlled by different feedback loops (each with different feedback signals as shown in FIG. 2). According to FIG. 2, there are first and second pump groups 202-1 and 201-2 controlled by first and second PID control circuits 204-1 and 204-2 which receive feedback via power splitter 203 feeding optical fibers OF1 and OF2 and diodes PD1 and PD2, forming different feedback paths for the two pump groups 202-1 and 202-2 shown. This second method has the capability to control the relatively slow gain transient due to counter-propagating signal-pump Raman interaction in a backward-pumped RFA but is not fast enough to control gain transients due to co-propagating signal-signal and signal-pump Raman interactions in a forward-pumped RFA. In addition, this second method requires a fast channel monitor to provide channel gain spectral information because each feedback loop requires an independent feedback signal. The need for a fast channel monitor not only complicates the amplifier design but also significantly increases cost of the design.
Recently, we, in addition to Mr. M. Feuer, have proposed and demonstrated a simple linear/log-linear feed-forward dynamic gain profile control technique for both a forward-pumped RFA and a backward-pumped RFA, for example, as described in several articles and in U.S. patent application Ser. Nos. 11/273,868 and 11/274,666, filed Nov. 15, 2005, and incorporated herein by reference as to its entire contents. The proposed technique allows us to control very fast gain transients due to both co-propagating signal-signal Raman interaction and co-propagating signal-pump Raman interaction in a forward-pumped RFA. The proposed technique also allows the control speed in a backward-pumped RFA to be accelerated due to the essence of the deterministic algorithm. In our recently filed U.S. patent application Ser. No. 11/424,312, filed, Jun. 15, 2006, we have extended the proposed Linear/Log-Linear feed-forward control technique from controlling a gain of a single RFA to control an overall gain of multiple cascaded RFAs by adjusting the pump powers of only one RFA or adjusting the pump power of more than one RFA but using only one feed-forward signal monitor. Finally, we have proposed that transient tilt control may be achieved using a fast dynamic gain control of an RFA in U.S. patent application Ser. No. 11/381,244, filed, May 2, 2006.
The proposed feed-forward control technique works well for a forward-pumped distributed/discrete RFA (FIG. 1(a)) as well as a backward-pumped discrete RFA (FIG. 1(b)), though, its advantage in the control of a backward-pumped distributed RFA is not so obvious because the location of the signal monitor and the location of the backward Raman pumps are geographically separated. Therefore, a telemetry channel is required to send the feed-forward signal to the control unit of the backward Raman pumps. For this case, a feedback-based control technique has an advantage because the feedback signal may be monitored right after the RFA. In addition, there is a chance that using a feed-forward dynamic gain control technique alone may not be enough for some ULH WDM system with higher performance/margin requirements. Consequently, one can see that there is still a real need in the art for a faster and more cost-effective feedback-based dynamic gain control technique.